In addition to reducing costs, a key challenge facing PV technologists is to improve the conversion efficiencies of solar cells. The greater the efficiency, the smaller the solar panel needs to be to produce a given output of energy.

Currently, PV efficiencies are pretty low; that’s why a solar system installed on a house rooftop can only supply a modest portion of the home’s electricity requirements. The typical crystalline silicon module in PV systems commonly installed today achieves about 15% efficiency: 15% of the energy from the solar radiation reaching the ground is turned into electricity. The theoretical maximum efficiency for solar is at least 65%, so there’s clearly a lot of room for improvement.

Earlier this month, Spectrolab — a wholly-owned subsidiary of Boeing (NYSE: BA) — announced that it had demonstrated a solar cell with 40.7% efficiency.

Until now, the best that had ever been achieved in a laboratory had been about 34% efficiency, so the Spectrolab announcement represents a significant leap forward.

When solar systems with efficiencies above 30% and costs below $2/watt can be produced in volume, we’ll be seeing large PV systems in fields and PV modules on most rooftops around the world — and solar can finally begin to fulfill its promise for sustainable energy supply.

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One note: Spectrolab's technology – triple junction Galium arsenide based cells – is ideally suited for concentrated PV systems. For the most part, concentrated modules are not ideal for rooftop solutions because of their height and weight (although some like solfocus are working on more streamlined concentrated modules).

Home's can today be powered through solar. By using south facing greenrooms for passive solar heating in the winter, using efficient insulation by constructing with Structural Insulated Panels, and placing passive solar water heaters on the roof in addition to solar panels, one can affordably and easily achieve a solar home. It's being done day. See demonstrations of the technology this fall in washington DC at the Solar Decathlon (sponsored by the DOE)and you'll see more than 14 such homes built in a day by university students from around the world.

During the time of house construction installing a water system is one of the prime question. These days people are looking for a resource that can make them energy independent. Solar hot water heaters are one from them. These Systems heat the water by harnessing the sun energy. It works on the principle of thermal conversion. Solar hot water systems are created to warmth the water for homes. One of the major part of solar hot water systems are solar water tank which collect the hot water for a long duration of time. These days more and more people are switching towards the solar hot water heaters not just because these systems are cost effective but these systems are eco-friend ally also.